NASA's Swift satellite recently detected a rising tide of high-energy X-rays from a source toward the center of our Milky Way galaxy. The outburst, produced by a rare X-ray nova, announced the presence of a previously unknown stellar-mass black hole.

"Bright X-ray novae are so rare that they're essentially once-a-mission events and this is the first one Swift has seen," said Neil Gehrels, the mission's principal investigator, at NASA's Goddard Space Flight Center in Greenbelt, Md. "This is really something we've been waiting for."

Video above: An X-ray outburst caught by NASA's Swift on Sept. 16, 2012,
resulted from a flood of gas plunging toward a previously unknown black
hole. Gas flowing from a sun-like star collects into a disk around the
black hole. Normally, this gas would steadily spiral inward. But in this
system, named Swift J1745-26, the gas collects for decades before
suddenly surging inward. Credit: NASA's Goddard Space Flight Center.

An X-ray nova is a short-lived X-ray source that appears suddenly, reaches its emission peak in a few days and then fades out over a period of months. The outburst arises when a torrent of stored gas suddenly rushes toward one of the most compact objects known, either a neutron star or a black hole.

The rapidly brightening source triggered Swift's Burst Alert Telescope twice on the morning of Sept. 16, and once again the next day.

Named Swift J1745-26 after the coordinates of its sky position, the nova is located a few degrees from the center of our galaxy toward the constellation Sagittarius. While astronomers do not know its precise distance, they think the object resides about 20,000 to 30,000 light-years away in the galaxy's inner region.

Ground-based observatories detected infrared and radio emissions, but thick clouds of obscuring dust have prevented astronomers from catching Swift J1745-26 in visible light.

The nova peaked in hard X-rays -- energies above 10,000 electron volts, or several thousand times that of visible light -- on Sept. 18, when it reached an intensity equivalent to that of the famous Crab Nebula, a supernova remnant that serves as a calibration target for high-energy observatories and is considered one of the brightest sources beyond the solar system at these energies.

Even as it dimmed at higher energies, the nova brightened in the lower-energy, or softer, emissions detected by Swift's X-ray Telescope, a behavior typical of X-ray novae. By Wednesday, Swift J1745-26 was 30 times brighter in soft X-rays than when it was discovered and it continued to brighten.

"The pattern we're seeing is observed in X-ray novae where the central object is a black hole. Once the X-rays fade away, we hope to measure its mass and confirm its black hole status," said Boris Sbarufatti, an astrophysicist at Brera Observatory in Milan, Italy, who currently is working with other Swift team members at Penn State in University Park, Pa.

The black hole must be a member of a low-mass X-ray binary (LMXB) system, which includes a normal, sun-like star. A stream of gas flows from the normal star and enters into a storage disk around the black hole. In most LMXBs, the gas in the disk spirals inward, heats up as it heads toward the black hole, and produces a steady stream of X-rays.

But under certain conditions, stable flow within the disk depends on the rate of matter flowing into it from the companion star. At certain rates, the disk fails to maintain a steady internal flow and instead flips between two dramatically different conditions -- a cooler, less ionized state where gas simply collects in the outer portion of the disk like water behind a dam, and a hotter, more ionized state that sends a tidal wave of gas surging toward the center.

NASA's Swift satellite. Credit: NASA's Goddard Space Flight Center.

"Each outburst clears out the inner disk, and with little or no matter falling toward the black hole, the system ceases to be a bright source of X-rays," said John Cannizzo, a Goddard astrophysicist. "Decades later, after enough gas has accumulated in the outer disk, it switches again to its hot state and sends a deluge of gas toward the black hole, resulting in a new X-ray outburst."

This phenomenon, called the thermal-viscous limit cycle, helps astronomers explain transient outbursts across a wide range of systems, from protoplanetary disks around young stars, to dwarf novae -- where the central object is a white dwarf star -- and even bright emission from supermassive black holes in the hearts of distant galaxies.

Swift, launched in November 2004, is managed by Goddard Space Flight Center. It is operated in collaboration with Penn State, the Los Alamos National Laboratory in New Mexico and Orbital Sciences Corp. in Dulles, Va., with international collaborators in the United Kingdom and Italy and including contributions from Germany and Japan.

Today, 5 October 2012, the European Southern Observatory (ESO) is celebrating 50 years since the signing of its founding convention. Over the last half century ESO has become the world’s most productive ground-based astronomical observatory. This morning, for the first time ever, observations with ESO’s Very Large Telescope were made of an object chosen by the public. The winner of an anniversary competition pointed the VLT towards the spectacular Thor’s Helmet Nebula and the observations were broadcast live over the internet. To mark the occasion ESO and its partners are organising many other activities in the 15 ESO Member States.

The signing of the ESO Convention on 5 October 1962 and the foundation of ESO was the culmination of the dream of leading astronomers from five European countries — Belgium, France, Germany, the Netherlands and Sweden. They had decided to join forces with the primary goal of building a large telescope that would give them access to the magnificent and rich southern sky.

The Thor’s Helmet Nebula (NGC 2359) in the constellation of Canis Major (The Great Dog)

"Fifty years later, the original hopes of the five founding members have not only become reality, but have been greatly surpassed," says Tim de Zeeuw, ESO’s Director General. "ESO has fully taken up the challenge of its mission to design, build and operate the most powerful ground-based observing facilities on the planet."

Operating three unique and world-class observing sites in Chile — La Silla, Paranal and Chajnantor — ESO has become a leader in the astronomical research community [1].

Wide-field view of the sky around the Thor’s Helmet Nebula

At Paranal, ESO operates the Very Large Telescope (VLT), the world’s most advanced visible-light astronomical observatory, which, since first light in 1998, has been a driving force in a new age of discoveries. On the Chajnantor Plateau in northern Chile, ESO and its international partners [2] are are building a revolutionary astronomical telescope — ALMA, the Atacama Large Millimeter/submillimeter Array [3] will help to unveil the mysteries of the cold Universe.

ESO’s original observatory at La Silla is still very productive and remains at the forefront of astronomical research. In particular the HARPS instrument on the 3.6-metre telescope is the world’s most successful exoplanet hunting machine.

Zooming in on Thor’s Helmet

ESO’s huge next telescope is only a few years away. The 39-metre European Extremely Large Telescope (E-ELT) will become "the world’s biggest eye on the sky". With first light planned for early in the next decade, the E-ELT will tackle the biggest scientific challenges of our time and may revolutionise our perception of the Universe as much as Galileo's telescope did more than 400 years ago.

To celebrate the 50th anniversary, ESO and its partners are organising many events and public initiatives during 2012 [4]. A series of special coordinated public events are taking place today in the 15 Member States, as well as a multitude of Awesome Universe exhibitions.

Panning across the Thor’s Helmet Nebula

As part of the anniversary celebrations, for the first time ever, this morning the VLT was pointed towards an object in the sky selected by members of the public — the Thor’s Helmet Nebula [5]. This nebula was picked in the recent Choose what the VLT Observes contest (ann12060). The observations were performed by Brigitte Bailleul — winner of the Tweet Your Way to the VLT! competition — and were broadcast live over the internet from Paranal Observatory. This image, taken in the superb conditions typical of Paranal, is the most detailed ever obtained of this striking object.

"With the VLT, ALMA and the future E-ELT, ESO is entering a new era, one that not even the initial bold dreams of ESO’s founding members could have anticipated. To all of you that have made it possible, on behalf of ESO, thank you!" concludes Tim de Zeeuw.

Notes:

[1] Information about the publication statistics at different observatories is given here.

[2] The ALMA project is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile.

[3] ALMA will be a single telescope composed of 66 high-precision antennas. ALMA's construction will be completed in 2013, but early scientific observations with a partial array began in 2011 (eso1137).

[4] A documentary movie is being released to celebrate the anniversary, together with a sumptuously illustrated book. The movie has been also released as episodes in ESO’s popular ESOcast video podcast series. In addition, a new and very detailed book on the history of ESO’s triumphs and challenges will be published.

[5] The Thor’s Helmet Nebula, also known as NGC 2359, lies in the constellation of Canis Major (The Great Dog). The helmet-shaped nebula is around 15 000 light-years away from Earth and is over 30 light-years across. The helmet is a cosmic bubble, blown as the wind from the bright, massive star near the bubble's centre sweeps through the surrounding molecular cloud. The glowing gas is heated by the energetic radiation provided by the central star. Many different colours, originating from different elements in the gas, are also visible, as well as many dust clouds.

More information:

The year 2012 marks the 50th anniversary of the founding of the European Southern Observatory (ESO). ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning the 39-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become "the world’s biggest eye on the sky".

jeudi 4 octobre 2012

A Delta IV Medium rocket in the (4,2) configuration launched today, October 4th 2012 at 12:10 UTC with the GPS IIF-3 spacecraft for the U.S. Air Force.

Launch of GPS IIF-3 on Delta IV Medium Rocket

Lift off occurred from Space Launch Complex 37 at Cape Canaveral.

The United Launch Alliance Delta 4 rocket will deploy the Air Force's third Block 2F navigation satellite for the Global Positioning System. The rocket will fly in the Medium+ (4,2) configuration with two solid rocket boosters. Delayed from Sept. 20.

Delta IV Medium rocket description

GPS IIF satellite by Boeing

GPS-2F (Global Positioning System) or Navstar-2F (Navigation System using Timing And Ranging) satellites are the fourth evolution stage of the second generation of the GPS satellites. Improvements included an extended design life of 12 years, faster processors with more memory, and a new civil signal on a third frequency.

GPS 2F-3 satellite

The GPS-2F satellites do not need to carry an apogee kick motor, in contrast to the earlier generations, as the launch vehicles provide direct insertion into the GPS orbit. Originally the Delta-4M version was to be used for the Delta launches, but a mass growth of the satellites required a switch to the more powerful Delta-4M+(4,2) version. For Atlas launches, the Atlas-5(401) version is used.

GPS constellation

Boeing was contracted with options for up to 33 Block-IIF satellites in 1996, but in 2001 the contract was reduced 12 Block-IIF satellites. In July 2006, satellites 10, 11 and 12 were contracted. The first Block-IIF satellite was originally scheduled to launch in 2006, but was finally launched in 2010.

Image above: NASA's Mars rover Curiosity cut a wheel scuff mark into a wind-formed ripple at the "Rocknest" site to give researchers a better opportunity to examine the particle-size distribution of the material forming the ripple. Image credit: NASA / JPL-Caltech.

NASA's Curiosity rover is in a position on Mars where scientists and engineers can begin preparing the rover to take its first scoop of soil for analysis.

Curiosity is the centerpiece of the two-year Mars Science Laboratory mission. The rover's ability to put soil samples into analytical instruments is central to assessing whether its present location on Mars, called Gale Crater, ever offered environmental conditions favorable for microbial life. Mineral analysis can reveal past environmental conditions. Chemical analysis can check for ingredients necessary for life.

"We now have reached an important phase that will get the first solid samples into the analytical instruments in about two weeks," said Mission Manager Michael Watkins of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Curiosity has been so well-behaved that we have made great progress during the first two months of the mission."

The rover's preparatory operations will involve testing its robotic scooping capabilities to collect and process soil samples. Later, it also will use a hammering drill to collect powdered samples from rocks. To begin preparations for a first scoop, the rover used one of its wheels Wednesday to scuff the soil to expose fresh material.

Next, the rover twice will scoop up some soil, shake it thoroughly inside the sample-processing chambers to scrub the internal surfaces, then discard the sample. Curiosity will scoop and shake a third measure of soil and place it in an observation tray for inspection by cameras mounted on the rover's mast. A portion of the third sample will be delivered to the mineral-identifying chemistry and mineralogy (CheMin) instrument inside the rover. From a fourth scoopful, samples will be delivered to both CheMin and to the sample analysis at Mars (SAM) instrument, which identifies chemical ingredients.

"We're going to take a close look at the particle size distribution in the soil here to be sure it's what we want," said Daniel Limonadi of JPL, lead systems engineer for Curiosity's surface sampling and science system. "We are being very careful with this first time using the scoop on Mars."

Image above: This patch of windblown sand and dust downhill from a cluster of dark rocks is the "Rocknest" site, which has been selected as the likely location for first use of the scoop on the arm of NASA's Mars rover Curiosity. Image credit: NASA / JPL-Caltech / MSSS.

The rinse-and-discard cycles serve a quality-assurance purpose similar to a common practice in geochemical laboratory analysis on Earth.

"It is standard to run a split of your sample through first and dump it out, to clean out any residue from a previous sample," said JPL's Joel Hurowitz, a sampling system scientist on the Curiosity team. "We want to be sure the first sample we analyze is unambiguously Martian, so we take these steps to remove any residual material from Earth that might be on the walls of our sample handling system."

Rocknest is the name of the area of soil Curiosity will test and analyze. The rover pulled up to the windblown, sandy and dusty location Oct. 2. The Rocknest patch is about 8 feet by 16 feet (2.5 meters by 5 meters). The area provides plenty of area for scooping several times. Diverse rocks nearby provide targets for investigation with the instruments on Curiosity's mast during the weeks the rover is stationed at Rocknest for this first scooping campaign.

Mars Science Laboratory (MSL). Image credit: NASA / JPL-Caltech

Curiosity's motorized, clamshell-shaped scoop is 1.8 inches (4.5 centimeters) wide, 2.8 inches (7 centimeters) long, and can sample to a depth of about 1.4 inches (3.5 centimeters). It is part of the collection and handling Martian rock analysis (CHIMRA) device on a turret of tools at the end of the rover's arm. CHIMRA also includes a series of chambers and labyrinths for sorting, sieving and portioning samples collected by the scoop or by the arm's percussive drill.

Following the work at Rocknest, the rover team plans to drive Curiosity about 100 yards (about 100 meters) eastward into the Glenelg area and select a rock as the first target for use of its drill.

JPL, a division of the California Institute of Technology, manages the Mars Science Laboratory Project and built Curiosity.

On 8 June, the high-resolution stereo camera on Mars Express captured a region within the 1800 km-wide and 5 km-deep Argyre basin, which was created by a gigantic impact in the planet’s early history.

After Hellas, the Argyre impact basin is the second largest on the Red Planet.

Argyre and Hooke Crater perspective view

The name stems from the Greek word ‘argyros’ (silver) and Argyre was an ‘island of silver’ in Greek and Roman mythology. Giovanni Schiaparelli, the famed Italian astronomer, gave the name to this bright region on Mars in his detailed 1877 map.

At the centre of the larger impact basin is a flat region known as Argyre Planitia. The Mars Express images in this release all show a portion of the northern part of this plain, with a large portion of each image dominated by the western half of the 138 km-wide Hooke Crater, named after the British physicist and astronomer Robert Hooke.

Argyre and Hooke Crater in context

Most of Argyre Planitia has been shaped by wind, glacial and lacustrine (lake-based) processes, creating the smoother appearance of the landscape surrounding Hooke Crater.

Inside Hooke Crater itself, prevailing wind activity has formed dunes and helped to create linear erosion features, clearly seen in the following topographic image.

Topographical view Argyre Planitia

The most striking feature of this image release, shown clearly in the first image at the top of the page, is the icing sugar-like covering of the surface to the south (left) of the image. This is frost, but made of carbon dioxide, not water.

Carbon dioxide ice is commonly seen on the surface of Mars, and has long been thought to form only at ground level, freezing out of the atmosphere as frost, which is most likely the case here.

3D anaglyph view Hooke Crater

The lowlands to the south (left in the first image) of Hooke and regions within the crater are covered by a thin ice layer. However, it is lacking on the inner north-facing crater wall. It was probably melted there by the Sun, as indicated by the timing of the image.

Taken at around 4:30 in the local afternoon and during the southern hemisphere’s mid-winter, the Sun would have been just over 20 degrees above the horizon. It should then have been able to melt ice on the steeper north-facing slopes, but would probably not have had enough time to warm and melt any on low-lying horizontal surfaces.

ESA's Mars Express

Schiaparelli would doubtless have marvelled at the exquisite images coming back from Mars Express, which continues to provide today’s scientists with a bounty of wonderful data.

mercredi 3 octobre 2012

Image above: The cosmic distance ladder, symbolically shown here in this artist's concept, is a series of stars and other objects within galaxies that have known distances. Image credit: NASA/JPL-Caltech.

Astronomers using NASA's Spitzer Space Telescope have announced the most precise measurement yet of the Hubble constant, or the rate at which our universe is stretching apart.

The Hubble constant is named after the astronomer Edwin P. Hubble, who astonished the world in the 1920s by confirming our universe has been expanding since it exploded into being 13.7 billion years ago. In the late 1990s, astronomers discovered the expansion is accelerating, or speeding up over time. Determining the expansion rate is critical for understanding the age and size of the universe.

Unlike NASA's Hubble Space Telescope, which views the cosmos in visible light, Spitzer took advantage of long-wavelength infrared light to make its new measurement. It improves by a factor of 3 on a similar, seminal study from the Hubble telescope and brings the uncertainty down to 3 percent, a giant leap in accuracy for cosmological measurements. The newly refined value for the Hubble constant is 74.3 plus or minus 2.1 kilometers per second per megaparsec. A megaparsec is roughly 3 million light-years.

"Spitzer is yet again doing science beyond what it was designed to do," said project scientist Michael Werner at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Werner has worked on the mission since its early concept phase more than 30 years ago. "First, Spitzer surprised us with its pioneering ability to study exoplanet atmospheres," said Werner, "and now, in the mission's later years, it has become a valuable cosmology tool."

In addition, the findings were combined with published data from NASA's Wilkinson Microwave Anisotropy Probe to obtain an independent measurement of dark energy, one of the greatest mysteries of our cosmos. Dark energy is thought to be winning a battle against gravity, pulling the fabric of the universe apart. Research based on this acceleration garnered researchers the 2011 Nobel Prize in physics.

"This is a huge puzzle," said the lead author of the new study, Wendy Freedman of the Observatories of the Carnegie Institution for Science in Pasadena. "It's exciting that we were able to use Spitzer to tackle fundamental problems in cosmology: the precise rate at which the universe is expanding at the current time, as well as measuring the amount of dark energy in the universe from another angle." Freedman led the groundbreaking Hubble Space Telescope study that earlier had measured the Hubble constant.

Glenn Wahlgren, Spitzer program scientist at NASA Headquarters in Washington, said infrared vision, which sees through dust to provide better views of variable stars called cepheids, enabled Spitzer to improve on past measurements of the Hubble constant.

"These pulsating stars are vital rungs in what astronomers call the cosmic distance ladder: a set of objects with known distances that, when combined with the speeds at which the objects are moving away from us, reveal the expansion rate of the universe," said Wahlgren.

Cepheids are crucial to the calculations because their distances from Earth can be measured readily. In 1908, Henrietta Leavitt discovered these stars pulse at a rate directly related to their intrinsic brightness.

This graph illustrates the Cepheid period-luminosity relationship, which scientists use to calculate the size, age and expansion rate of the universe. Image credit: NASA/JPL-Caltech/Carnegie.

To visualize why this is important, imagine someone walking away from you while carrying a candle. The farther the candle traveled, the more it would dim. Its apparent brightness would reveal the distance. The same principle applies to cepheids, standard candles in our cosmos. By measuring how bright they appear on the sky, and comparing this to their known brightness as if they were close up, astronomers can calculate their distance from Earth.

Spitzer observed 10 cepheids in our own Milky Way galaxy and 80 in a nearby neighboring galaxy called the Large Magellanic Cloud. Without the cosmic dust blocking their view, the Spitzer research team was able to obtain more precise measurements of the stars' apparent brightness, and thus their distances. These data opened the way for a new and improved estimate of our universe's expansion rate.

"Just over a decade ago, using the words 'precision' and 'cosmology' in the same sentence was not possible, and the size and age of the universe was not known to better than a factor of two," said Freedman. "Now we are talking about accuracies of a few percent. It is quite extraordinary."

Pristine material that matches comets in our own Solar System have been found in a dust belt around the young star Beta Pictoris by ESA’s Herschel space observatory.

Twelve-million-year-old Beta Pictoris resides just 63 light-years from Earth and hosts a gas giant planet along with a dusty debris disc that could, in time, evolve into a torus of icy bodies much like the Kuiper Belt found outside the orbit of Neptune in our Solar System.

Olivine crystals

Thanks to the unique observing capabilities of Herschel, the composition of the dust in the cold outskirts of the Beta Pictoris system has been determined for the first time.

Of particular interest was the mineral olivine, which crystallises out of the protoplanetary disc material close to newborn stars and is eventually incorporated into asteroids, comets and planets.

“As far as olivine is concerned, it comes in different ‘flavours’,” explains Ben de Vries from KU Leuven and lead author of the study reported in Nature.

“A magnesium-rich variety is found in small and primitive icy bodies like comets, whereas iron-rich olivine is typically found in large asteroids that have undergone more heating, or ‘processing’.”

Herschel detected the pristine magnesium-rich variety in the Beta Pictoris system at 15–45 astronomical units (AUs) from the star, where temperatures are around –190ºC.

For comparison, Earth lies at 1 AU from our Sun and the Solar System’s Kuiper Belt extends from the orbit of Neptune at about 30 AU out to 50 AU from the Sun.

Beta Pictoris system

The Herschel observations allowed astronomers to calculate that the olivine crystals make up around 4% of the total mass of the dust found in this region.

In turn, this finding led them to conclude that the olivine was originally bound up inside comets and released into space by collisions between the icy objects.

“The 4% value is strikingly similar to that of Solar System comets 17P/Holmes and 73P/Schwassmann-Wachmann 3, which contain 2–10% magnesium-rich olivine,” says Dr de Vries.

“Since olivine can only crystallise within about 10 AU of the central star, finding it in a cold debris disc means that it must have been transported from the inner region of the system to the outskirts.”

The ‘radial mixing’ transport mechanism is known from models of the evolution of swirling protoplanetary discs as they condense around new stars.

The mixing is stimulated in varying amounts by winds and heat from the central star pushing materials away, along with temperature differences and turbulent motion created in the disc during planet formation.

Herschel

“Our findings are an indication that the efficiency of these transport processes must have been similar between the young Solar System and within the Beta Pictoris system, and that these processes are likely independent of the detailed properties of the system,” says Dr de Vries.

Indeed, Beta Pictoris is over one and a half times the mass of our Sun, eight times as bright, and its planetary system architecture is different to our own Solar System today.

“Thanks to Herschel, we were able to measure the properties of pristine material left over from the initial planet-building process in another solar system with a precision that is comparable to what we could achieve in the laboratory if we had the material here on Earth,” says ESA’s Herschel project scientist Göran Pilbratt.

Image above: A dying star is throwing a cosmic tantrum in this combined image from NASA's Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX), which NASA has lent to the California Institute of Technology in Pasadena. Image credit: NASA/JPL-Caltech.

A dying star is refusing to go quietly into the night, as seen in this combined infrared and ultraviolet view from NASA's Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX), which NASA has lent to the California Institute of Technology in Pasadena. In death, the star's dusty outer layers are unraveling into space, glowing from the intense ultraviolet radiation being pumped out by the hot stellar core.

Artist's impression of GALEX. Credit: NASA.

This object, called the Helix nebula, lies 650 light-years away in the constellation of Aquarius. Also known by the catalog number NGC 7293, it is a typical example of a class of objects called planetary nebulae. Discovered in the 18th century, these cosmic works of art were erroneously named for their resemblance to gas-giant planets.

Planetary nebulae are actually the remains of stars that once looked a lot like our sun. These stars spend most of their lives turning hydrogen into helium in massive runaway nuclear fusion reactions in their cores. In fact, this process of fusion provides all the light and heat that we get from our sun. Our sun will blossom into a planetary nebula when it dies in about five billion years.

Spitzer Space Telescope. Image credit: NASA / JPL-Caltech.

When the hydrogen fuel for the fusion reaction runs out, the star turns to helium for a fuel source, burning it into an even heavier mix of carbon, nitrogen and oxygen. Eventually, the helium will also be exhausted, and the star dies, puffing off its outer gaseous layers and leaving behind the tiny, hot, dense core, called a white dwarf. The white dwarf is about the size of Earth, but has a mass very close to that of the original star; in fact, a teaspoon of a white dwarf would weigh as much as a few elephants!

The intense ultraviolet radiation from the white dwarf heats up the expelled layers of gas, which shine brightly in the infrared. GALEX has picked out the ultraviolet light pouring out of this system, shown throughout the nebula in blue, while Spitzer has snagged the detailed infrared signature of the dust and gas in red, yellow and green. Where red Spitzer and blue GALEX data combine in the middle, the nebula appears pink. A portion of the extended field beyond the nebula, which was not observed by Spitzer, is from NASA's all-sky Wide-field Infrared Survey Explorer (WISE). The white dwarf star itself is a tiny white pinprick right at the center of the nebula.

mardi 2 octobre 2012

Image above: NASA's Mars rover Curiosity held its Mars Hand Lens Imager (MAHLI) camera about 10.5 inches (27 centimeters) away from the top of a rock called "Bathurst Inlet" for a set of eight images combined into this merged-focus view of the rock. This context image covers an area roughly 6.5 inches by 5 inches (16 centimeters by 12 centimeters). Resolution is about 105 microns per pixel.

MAHLI took the component images for this merged-focus view, plus closer-up images of Bathurst Inlet, during Curiosity's 54th Martian day, or sol (Sept. 30, 2012). The instrument's principal investigator had invited Curiosity's science team to "MAHLI it up!" in the selection of Sol 54 targets for inspection with MAHLI and with the other instrument at the end of Curiosity's arm, the Alpha Particle X-Ray Spectrometer.

A merged-focus MAHLI view from closer to the rock, providing even finer resolution, is at http://photojournal.jpl.nasa.gov/catalog/14763 .

The Bathurst Inlet rock is dark gray and appears to be so fine-grained that MAHLI cannot resolve grains or crystals in it. This means that the grains or crystals, if there are any at all, are smaller than about 80 microns in size. Some windblown sand-sized grains or dust aggregates have accumulated on the surface of the rock but this surface is clean compared to, for example, the pebbly substrate below the rock (upper left and lower right in this context image).

MAHLI can do focus merging onboard. The full-frame versions of the eight separate images that were combined into this view were not even returned to Earth -- just the thumbnail versions. Merging the images onboard reduces the volume of data that needs to be downlinked to Earth.

Mars Streambed

Video above: Curiosity science team member Sanjeev Gupta explains how rounded pebbles spotted by the rover are convincing evidence of an ancient streambed on Mars. Credit: NASA/JPL-Caltech.

NASA's Mars rover Opportunity, well into its ninth year on Mars, will work for the next several weeks or months at a site with some of the mission's most intriguing geological features.

The site, called "Matijevic Hill," overlooks 14-mile-wide (22-kilometer-wide) Endeavour Crater. Opportunity has begun investigating the site's concentration of small spherical objects reminiscent of, but different from, the iron-rich spheres nicknamed "blueberries" at the rover's landing site nearly 22 driving miles ago (35 kilometers).

The small spheres at Matijevic Hill have different composition and internal structure. Opportunity's science team is evaluating a range of possibilities for how they formed. The spheres are up to about an eighth of an inch (3 millimeters) in diameter.

The "blueberries" found earlier are concretions formed by the action of mineral-laden water inside rocks, but that is only one of the ways nature can make small, rounded particles. One working hypothesis, out of several, is that the new-found spherules are also concretions but with a different composition. Others include that they may be accretionary lapilli formed in volcanic ash eruptions, impact spherules formed in impact events, or devitrification spherules resulting from formation of crystals from formerly melted material. There are other possibilities, too.

"Right now we have multiple working hypotheses, and each hypothesis makes certain predictions about things like what the spherules are made of and how they are distributed," said Opportunity's principal investigator, Steve Squyres, of Cornell University, Ithaca, N.Y. "Our job as we explore Matijevic Hill in the months ahead will be to make the observations that will let us test all the hypotheses carefully, and find the one that best fits the observations."

The team chose to refer to this important site as Matijevic Hill in honor of Jacob Matijevic (1947-2012), who led the engineering team for the twin Mars Exploration Rovers Spirit and Opportunity for several years before and after their landings. He worked at NASA's Jet Propulsion Laboratory, Pasadena, Calif., from 1981 until his death last month, most recently as chief engineer for surface operations systems of NASA's third-generation Mars rover, Curiosity. In the 1990s, he led the engineering team for the first Mars rover, Sojourner.

Mars Exploration Rover B "Opportunity" (MER-B)

A different Mars rover team, operating Curiosity, has also named a feature for Matijevic: a rock that Curiosity recently investigated about halfway around the planet from Matijevic Hill.

Opportunity's project manager, John Callas, of JPL, said, "If there is one person who represents the heart and soul of all three generations of Mars rovers -- Sojourner, Spirit and Opportunity, Curiosity -- it was Jake."

lundi 1 octobre 2012

Venus Express has spied a surprisingly cold region high in the planet’s atmosphere that may be frigid enough for carbon dioxide to freeze out as ice or snow.

The planet Venus is well known for its thick, carbon dioxide atmosphere and oven-hot surface, and as a result is often portrayed as Earth’s inhospitable evil twin.

But in a new analysis based on five years of observations using ESA’s Venus Express, scientists have uncovered a very chilly layer at temperatures of around –175ºC in the atmosphere 125 km above the planet’s surface.

The curious cold layer is far frostier than any part of Earth’s atmosphere, for example, despite Venus being much closer to the Sun.

Venus terminator

The discovery was made by watching as light from the Sun filtered through the atmosphere to reveal the concentration of carbon dioxide gas molecules at various altitudes along the terminator – the dividing line between the day and night sides of the planet.

Armed with information about the concentration of carbon dioxide and combined with data on atmospheric pressure at each height, scientists could then calculate the corresponding temperatures.

“Since the temperature at some heights dips below the freezing temperature of carbon dioxide, we suspect that carbon dioxide ice might form there,” says Arnaud Mahieux of the Belgian Institute for Space Aeronomy and lead author of the paper reporting the results in the Journal of Geophysical Research.

Clouds of small carbon dioxide ice or snow particles should be very reflective, perhaps leading to brighter than normal sunlight layers in the atmosphere.

“However, although Venus Express indeed occasionally observes very bright regions in the Venusian atmosphere that could be explained by ice, they could also be caused by other atmospheric disturbances, so we need to be cautious,” says Dr Mahieux.

Terminator temperature profile

The study also found that the cold layer at the terminator is sandwiched between two comparatively warmer layers.

“The temperature profiles on the hot dayside and cool night side at altitudes above 120 km are extremely different, so at the terminator we are in a regime of transition with effects coming from both sides.

“The night side may be playing a greater role at one given altitude and the dayside might be playing a larger role at other altitudes.”

Similar temperature profiles along the terminator have been derived from other Venus Express datasets, including measurements taken during the transit of Venus earlier this year.

Models are able to predict the observed profiles, but further confirmation will be provided by examining the role played by other atmospheric species, such as carbon monoxide, nitrogen and oxygen, which are more dominant than carbon dioxide at high altitudes.

“The finding is very new and we still need to think about and understand what the implications will be,” says Håkan Svedhem, ESA’s Venus Express project scientist.

“But it is special, as we do not see a similar temperature profile along the terminator in the atmospheres of Earth or Mars, which have different chemical compositions and temperature conditions.”

Earth observation measurements shouldn’t be taken with a pinch of salt. ESA is comparing readings of sea-surface salinity from drifting floats to confirm the SMOS water mission’s measurements.

Since its launch in 2009, ESA’s Soil Moisture and Ocean Salinity (SMOS) satellite has been helping us to understand the water cycle.

As with any Earth observation mission, it is important to validate the readings acquired from space. This involves comparing the satellite data with measurements taken directly in the water.

SMOS vs Argo

For SMOS, that means comparing its readings to data from floats or drifters that measure ocean salinity at different depths.

One of the major networks of in-situ drifters is Argo. The network, involving over 50 research and operational agencies in more than 30 countries, uses autonomous floats to collect temperature, salinity and deep current data.

With over 3500 active drifters, the Argo floats acquire in situ data in the upper 2000 m of the ocean.

These measurements are then directly compared to SMOS data, which in turn cover the global ocean and provide measurements of the salinity in the first centimetre of the sea surface.

Argo floats (click on the image for enlarge)

SMOS provides measurements averaged over a surface of 40x40 sq km, but the difference of the size of the area measured and other influencing factors like background noise lead to differences between SMOS and Argo measurements.

“Since Argo measurements are taken much deeper than SMOS’s, the stratification of the upper layer of the ocean needs to be taken into account when comparing the two salinities in rainy regions,” said Jacqueline Boutin from France’s Laboratory for Oceanography and Climate (LOCEAN).

Argo floats

“For example, rain over the ocean will cause SMOS to pick up lower salinity readings than Argo.”

The advantage that SMOS has over the Argo floats is that the satellite provides a complete view of the global ocean every five days.

Argo measurements, on the other hand, provide punctual salinity data sampled at a lower resolution than SMOS every 10 days.

The higher precision provided by the Argo floats, however, complements the SMOS measurements.